The present invention relates generally to tuning of a resistor in an RC filter, and is particularly concerned with tuning of on-chip, microelectronic components.
Filtering is a fundamental signal processing tool used in almost all electronics systems. While some filtering functions can be performed in the digital domain, filtering in analog domain is essential in countless applications where only analog techniques enable high-speed processing of small signal levels with the required resolution. Front-end processing in almost all electronic systems such as wireless and wireline communications, video, audio, imaging, etc. rely heavily on filtering in the analog domain.
We have been witnessing in the last several decades that the cost and footprint of electronic systems are scaling down significantly as increasingly more functionality is integrated on a single chip of semiconductor. While microchip processing is extremely efficient in building large number of devices on a microchip, analog processing still suffers from variations in component values from one fabrication run to another. Further limitations to analog processing accuracy come from the dependence of component parameters on temperature, which is becoming more and more costly to control as a result of large scale integration. A widely recognized solution to this problem is the tuning of on-chip components until the error due to component variations becomes negligible.
In a typical RC filter composed of amplifiers, resistors and capacitors, the accuracy of filter transfer function is determined by the resistor and capacitor values. In general, either resistors or capacitors can be tuned to obtained the required overall transfer function. Tuning can be performed efficiently by switching small-valued components in or out of the circuit. As the dominant non-ideality of a reasonably-sized micro-switch is its parasitic resistance (up to very high frequencies), tuning of capacitors poses a difficulty as a result of an undesired resistance appearing in series with the capacitor to be tuned. Tuning of resistors, therefore, can be more effective in many realizations.
FIG. 1 illustrates a conventional scheme of tuning a resistor via switching in or out some small-valued resistors that are in series with the resistor to be tuned. In this circuit, switches are closed and opened to include more or less resistance Rx in series with the resistance to be tuned (Rtune). When fine tuning is desired, the switched resistors need to be much smaller than the resistor to be tuned, typically on the order of one hundredth or less. This necessarily requires large size switches, such that the parasitic switch resistance can be neglected next to the tuning resistors. A typical example is a 5 k resistor to be tuned to below 1% precision. This requires less than 50 ohm tuning resistors, which in turn requires a switch resistance on the order of 10 ohms or less. A switch with such a low on resistance requires a transistor that is several hundred times larger than a minimum geometry device. Note that, increased switch size, besides requiring more chip real estate, also exhibits higher parasitic capacitance along the signal path, and increased noise coupling through the substrate. The large spread of resistor values also limits the accuracy and matching between resistors. Small-valued resistors also require much more hand-tailoring in layout, as their aspect ratios turn out to be awkward, and parasitic contact resistances introduce considerable error to overall resistance. Another difficulty is that different tuning resistor values are needed for each different resistor value to be tuned (such that the same relative accuracy can be maintained across, all resistors). For example, 50 ohm resistors are needed to tune a 5 k resistance, whereas 75 ohm resistors would be needed to tune 7.5 k resistor with the same relative increments.
It is an object of the present invention to provide a new and improved filter tuning network which is particularly suitable for tuning microchip integrated resistors.
According to one aspect of the present invention, a resistor tuning network for connection in series with a resistor to be tuned is provided, which comprises a first resistor having a fixed resistance and ladder circuit comprising a resistor ladder network connected in parallel with the first resistor, the ladder circuit having an input and first and second output terminals, a plurality of arms connected in series with the input and first output terminal, each arm having a resistor R and a node separating each pair of adjacent arms, a shunt arm connected to each node, each shunt arm having a resistor which has a value which is a multiple of R and a switch in series, each switch having a first, closed position connecting the shunt arm to the first output terminal and a second, open position connecting the shunt arm to the second output terminal, such that the resistance of the ladder network is varied dependent on the switch positions and is at a maximum value when all switches are closed and a minimum value when all switches are open, whereby a resistance can be tuned by varying the switch positions in the ladder circuit.
This arrangement produces a tuning network which exhibits good linearity in the resistor value with switch positions, allowing fine tuning of a large spread of resistor values. In an exemplary embodiment, a third resistor R3 is connected in series with the tuning network of the first resistor and ladder circuit. The ladder network may be an R-2R ladder. Because the switches in the R-2R ladder are all in series with the same amount of resistance, i.e. 2R, they can be uniformly sized. In an exemplary embodiment, 2R is of the order of at least one kilohm, so that switch sizes can be kept small and switch induced non-linearities can be kept negligible. Thus, this arrangement permits an effective tuning network for an RC filter to be integrated on a single chip.
In an exemplary embodiment of the invention, the equivalent resistance REQ of the ladder network is given by the following relationship:   REQ  =            1      +              R3        ⁢                  xe2x80x83                ⁢                  (                                    R1                              -                1                                      +                          R2                              -                1                                              )                                    R1                  -          1                    +                        b                      -            1                          ⁢                  R2                      -            1                              
where R1 is the first resistance connected in parallel with the R-2R ladder circuit, R2 is the resistance of the R-2R ladder circuit, which is varied according to the number of switches which are closed, R3 is the third resistance, and b represents the current switch positions and is controlled by the function:   b  =            ∑              i        =        1            n        ⁢          xe2x80x83        ⁢                  C        i            ⁢              xe2x80x83            ⁢              2                  -          i                    
Where n is the number of branches in the R-2R ladder.
According to another aspect of the present invention, a method of tuning the resistance of an RC filter in a microchip is provided, which comprises the steps of:
connecting a resistor tuning network including an R-xR ladder in series with a resistor to be tuned which forms part of an RC filter, where xR is a multiple of R; and
varying the resistance of the R-xR ladder by selectively opening and closing switches connected in xR branches of the R-xR ladder to connect lesser or greater numbers of the xR branches to an output terminal of the R-xR ladder in order to tune the resistor to be tuned to a desired resistance value.
In an exemplary embodiment, the resistor tuning network is integrated on a microchip on which the RC filter is built. In order to improve linearity of the tuning transfer function with successive switch openings along the ladder, a tuning network comprising the R-xR ladder in parallel with a fixed resistance is connected in series with a third resistor. The R-xR ladder is an R-2R ladder in an exemplary embodiment of the invention.
By connecting a resistance in parallel with an R-2R ladder circuit, a resistor tuning network is provided which has good linearity in its tuning transfer function and allows a large spread of resistor values. With this arrangement, high tuning accuracy is achieved with a small spread in resistor values. Because the switches in the R-2R ladder are all in series with the same amount of resistance (i.e. 2R), they can be uniformly sized. By selecting 2R to be of the order of 1 kohm or more, switch sizes can be made small, enabling the network to be integrated into a microelectronic chip or silicon chip.